I test the rMT in healthy participants measuring MEPs over separate days, pre and post rTMS. I would like to know how much it can fluctuate normally and how can I test whether the post-tms rMT is significantly different than pre-tms rMT.
Hi Stefania,
We've done a few TMS reliability studies and the answer to the question is, of course, somewhat complicated. There are a number of factors to consider with respect to your participant demographics. Aging has a strong influence on motor threshold reliability (see McGregor et al., 2012), so if you are crossing age groups, it is relevant to note that your older participants will have less session-to-session reliability. There are numerous other factors that can affect session to session reliability of rMT. These include wakefulness/circadian cycles, hormone control, cortisol level, physical fitness, and dual task influence (among many others).
That said, you can attempt to control for those variables with careful experimental design and fairly rigid scheduling with your participants. However, rMT can even vary within session by up to 5-10% MSO or even more if your participants are prone to get sleepy. As such, I would recommend using recruitment curves (I/O curves) for cortical excitability, as these tend to vary less when used with a fixed reference stimulation (i.e. - specific MSO values not relative to subjects).
That sounds very doom-and-gloom, but it's just been my experience with 7+ years of TMS. It is NOT to say that you should stop using TMS for outcome assessment after treatment, however. The work is EXTREMELY valuable to the motor control and rehabilitation community.
Dear Keith,
thank you very much for your answer. I measure rMT on healthy young adults to perform rTMS at 90% MSO of their rMT. I then re-test their rMT at the end of the experiment (2 hours later) and I have noticed some changes. More specifically, I find an increase in % of MSO necessary to elicit the twitches (reduced rMT). I reasoned this might be due to fatigue or sleepiness, but also since I retest the same subject over 3 separate days I was curious to know whether there is literature about how stable is the rMT over time. Again, thank you for your response!
Stefania
Dear Stefania, the literature on test-retest repeatibility of resting motor threshold indicates considerable variability, around 10-15%
Papers that specifically addressed this issue are:
Mills and Nithi - Muscle Nerve 1997
Conforto, Z'Graggen, Kohl et al. - Clin Neurophysiol 2004
Wassermann - Clin Neurophysiol 2002
Kimiskidis et al. - Neurophysiologie Clinique 2004
Badawy et al. - Neurophysiologie Clinique 2011
Oliviero et al. - Neruoscience Research 2006
Koski et al. - Clin Neurophysiol 2005
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